DAT40 on rp3410 - 7 day problem

[Michael Poland_3]

We have recently been experiencing a problem with backups to the external DAT40 device.
Daily system backups usually take around 90 mins to complete.
In the last 5 weeks or so, the Tuesday night backup has been taking much longer although the amount of data reported as stored is still the same.
Also the clean light comes on during the back up.
The odd thing here is that it happens only on a tuesday night.
We have tried cleaning the drive on a tuesday, and using different tapes that work fine on other days. I've powered the unit down and back up again. But still, Wednesday morning same thing , clean light flashing and log files show backup took an excessive time to complete.
Any ideas please?
Backups are down using fbackup, OS is hpux 11.11
Many thanks.